The present invention relates to a safety device for a sealed cell. It also covers a method of manufacturing such a device.
A sealed electrochemical cell (or “battery”) comprises in conventional manner an electrochemical stack made up of alternating positive and negative electrodes on either side of separators impregnated in electrolyte. Each electrode is made up of a metal current collector supporting an electrochemically active material on at least one of its faces. Each electrode is electrically connected to a current output which provides electrical continuity between the electrode and an external application with which the cell is associated. The stack of electrodes is placed in a container that is closed in sealed manner by a cover.
The invention relates to sealed cells of the nickel cadmium, nickel metal hydride, and lithium ion types. These cells may be of cylindrical format or of rectangular format (also known as prismatic format). They constitute cells that are rechargeable, also known as secondary cells or as accumulators.
A cell is generally designed to operate under so-called “nominal” conditions, i.e. within given ranges of temperature, current, and voltage. When a sealed cell is used outside the nominal conditions, for example in the event of accidental overcharging, or a short circuit, or temperature higher than the maximum operating temperature, etc., there is a danger of explosion. This is because such situations lead to the electrolyte heating and to electrolyte vapor being formed. An accumulation of such vapor inside the container increases the internal pressure of the cell and that can lead to the container bursting violently and to chemical compounds being sprayed out that are harmful and corrosive for the environment and for people situated nearby.
Safety devices exist that prevent gas from accumulating inside the container of a sealed cell and that enable the gas to be exhausted when the internal pressure exceeds a predetermined value.
Known safety devices are generally constituted by valves. U.S. Pat. No. 5,523,178 describes a valve for a cell. That valve nevertheless presents the drawback of being complex in design. When valves are simple in design they suffer from the drawback of opening only at pressures that are high or opening only over a narrow range of pressures.
Furthermore, it is necessary for a cell safety device also to possess a circuit interrupter function suitable for electrically and irreversibly isolating apparatus connected to the cell. Known valves do not perform this circuit interrupter function. A specific electrical device must be provided for performing the circuit interrupter function when the valve is activated.
There therefore exists a need for a safety device for a sealed cell which is simple in design, suitable for low pressures, and capable of operating over a wide range of pressures. There also exists a need for a safety device that acts simultaneously as a circuit interrupter.
An object of the present invention is to solve the above-mentioned problems.